1. Field of the Invention
The present invention relates generally to the field of telecommunications. More specifically, the present invention is directed to a receiver comprising analog signal processing means, intermodulation interference detection means and an automatic gain control circuit (AGC) for controlling the gain of said analog signal processing means, and the gain of said RF analog signal processing means.
2. Description of the Related Art
A conventional design for such a receiver is known from U.S. Pat. No. 4,654,884. The radio receiver disclosed therein, comprises an automatic gain control circuit in which intermodulation interference is detected by comparing the signal level within the bandwidth of a desired radio channel with that of interfering radio-channels deviating from the desired radio-channel by a frequency distance allowing intermodulation interferences to occur within the desired radio-channel. An intermodulation interference indication signal is generated when the signal level of the desired radiochannel becomes smaller than the signal level of one of the interfering channels. The intermodulation interference indication signal is used to control the gain of the RF analog signal processing means.
The automatic gain control obtained therewith requires accurate matching between the specific characteristic of the intermodulation interference indication signal being generated on the one hand and the deterioration of the useful signal caused by the initiating intermodulation interference on the other hand. Due to element tolerance deviations a safety margin has to be taken into account in defining the specific characteristic of the intermodulation interference indication signal to prevent either over-or underdosing the gain control, respectively resulting in loss of useful signal components or insufficient suppression of intermodulation interference. This safety margin thus limits the accuracy in gain control.
Furthermore, to measure the signal level of an interfering channel, the receiver has to interrupt its correct tuning position, in which a desired radio-signal is received, and switch over its tuning to the interfering channel. To eliminate the switching of the tuning and the reception of interfering signals during level detection, the receiver has to be muted. To come to a reliable level detection, the period during which the receiver has to stay tuned to said interfering channel has to exceed a certain level detection time constant, thereby defining a minimum duration for the mute period. Furthermore, especially when the receiver is used in a fast changing environment, e.g. when being mounted on a moving object such as an automobile, the interference channel level check has to be carried out rather frequently, thereby requiring a minimum repetition frequency for the mute actions. This all significantly deteriorates the overall receiver performance.
Therefore, it is a first object of the invention to provide a receiver with an automatic gain control circuit which eliminates intermodulation interference more accurately than the receiver disclosed in the above referenced U.S. patent. A second object of the invention is to provide a receiver with an automatic gain control circuit without the necessity to interrupt the processing of the desired signal and switch the tuning. Other objects and advantages of the present invention will become apparent from the following summary and detailed description.
A receiver of one of the referenced types according to the invention is therefore characterized in that an output of the analog signal processing means is coupled through frequency selective means for channel selection and an amplitude detector to a first terminal of the interfering signals detection means. A signal generator for generating a gain modulation signal is coupled to a gain control input of said analog signal processing means as well as to a second terminal of the interfering signal detection means. The intermodulation interference detection means comprises a correlator included in a feed back loop of said automatic gain control circuit. It supplies a DC gain control signal input of the analog signal processing means for detection of differences in relative variations between the gain control signal on the one hand and the output signal of the amplitude detector of the other hand for reducing said differences by a DC gain setting adjustment of the analog signal processing means.
A variation of the gain of the analog signal processing means will cause the level of the useful signal to vary linearly with said gain variation, whereas intermodulation interference components and harmonic distortion products will vary to a second or higher order with said gain variation. According to the invention, this difference in gain variation is used to detect whether the output signal level of the analog signal processing means is predominated by the useful signal or by intermodulation and/or harmonic distortion interferences. The occurrence of a signal level at the output of the analog signal processing means varying disproportionally, i.e. to a second or higher order, with a gain variation of the analog signal processing means, is accurately detected in the correlator, and fed back as a gain control signal to the RF analog signal processing means. Due to this feedback, the accuracy in gain control is immune from element tolerances.
Furthermore, the detection of the intermodulation and/or harmonic distortion interferences is applied during correct tuning at a desired radio-signal, which, in contrast to the above mentioned known method of intermodulation interference detection, does not require to change tuning and mute the receiver. In the following the description given with respect to the detection and elimination of intermodulation interference also applies to harmonic distortion interferences.
A preferred embodiment of a receiver according to the invention is characterized in that the gain modulation signal comprises a bandwidth limited pseudo random carrier frequency. The use of such gain control signal prevents the gain variation from becoming noticeable in the useful signal. For the same purpose, the amplitude of the gain modulation signal is preferably chosen to be smaller than 2 dB.
Another preferred embodiment of a receiver according to the invention, in which the effect of the gain variation on the signal output level of the analog signal processing means is prevented from becoming noticeable in the useful signal, is characterized in that the signal generator is coupled through inverting means to a modulation signal input terminal of an amplitude modulator following said analog signal processing means. With this measure, said gain variations can be completely eliminated.
Preferably, a delay circuit is coupled between the signal generator and said modulation signal input terminal of the amplitude modulator to compensate for the signal delay occurring in the analog signal processing means.
For a cost effective realization, the analog signal processing means preferably comprises a controllable attenuator preceding a broadband amplifying stage coupled to the gain control input of the analog signal processing means providing a signal attenuation varying with said gain control signal. In a further preferred embodiment, the frequency selection means for channel selection comprises a mixing stage followed by an IF circuit for IF channel selection. An output of said IF circuit is coupled through said level detecting means to the first terminal of the correlator.
Another preferred embodiment of a receiver according to the invention is characterized in that an output of the signal generator is coupled to a first input of a signal combiner, an output of the correlator being coupled to a second input of the signal combiner and an output of said signal combiner being coupled through said gain control input of the analog signal processing means to said controllable attenuator.
The above and other objects, features and advantages of the present invention will become more apparent from the following description referring to the accompanying drawings, in which a preferred embodiment of the present invention is shown by way of illustrative example and in which identical reference indications are used for corresponding circuitry.